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Tensile properties of cold rolled AISI 316L stainless steel after full reversion of martensite to austenite, recrystallization of retained austenite, and grain growth were studied at 850, 950, and 1050 °C. At higher temperatures, it was found that the kinetics of the reversion and recrystallization processes enhance but coarser grain sizes will be obtained at the end of recrystallization. At 1050 °C, appreciable grain growth was observed after the completion of the recrystallization process, which was not the case for a low temperature of 850 °C. At the stage of full recrystallization, by decreasing the annealing temperature, the yield stress (YS) and the ultimate tensile strength (UTS) values increased and total elongation decreased, which was related to the grain size strengthening by the Hall-Petch law. However, the Hall-Petch slope for the UTS was found to be much smaller than that of YS, which reveals that YS has greater grain size dependency. The latter was ascribed to the improved work-hardening behavior and enhanced transformation-induced plasticity (TRIP) effect by coarsening of grain size. To obtain high-strength and ductile steel with tensile toughness higher than 300 MJ/m3 and yield ratio of ∼0.5, the average grain size of ∼3 μm was found to be desirable.

International audience; In the present article, metallic nanoparticles (NPs) used as nanogauges and electron backscattered diffraction (EBSD) are combined to investigate deformation mechanisms of the UR45 N duplex steel. The kinematic fields bring precious details in the evolution of the local deformation of austenitic and ferritic phases. The crystallographic information at the initial stage and in the plastic domain is obtained by the EBSD technique. Strain evolutions in each phase are analyzed. The first strain marks appear in austenite just before the elastoplastic transition. Slip and twinning activities in austenite constitute an important part of the work hardening of the material. The slips in ferrite are visible in the plastic domain and present complex geometry. The role of phase boundaries, which act as strain barrier is highlighted. The incompatibilities of deformations between the two phases modify the deformation mechanism of austenite in comparison with a pure austenitic stainless steel as 316 L. Local interactions between phases during the deformation are analyzed. The strong strain heterogeneities in the plastic domain are analyzed in the kinematic fields. Local evolutions of the deformation are also confronted to the macroscopic behavior of the material.

The stacking fault energy (SFE) is often used as a key parameter to predict and describe the mechanical behaviour of face centered cubic material. The SFE determines the width of the partial disloc ...

A Cr–Mn–Ni–N series high strength metastable austenitic stainless steel (14Cr10Mn) was used to investigate the effect of cold rolling reduction on mechanical properties at various low testing temperatures. Microstructures of tested samples were observed by transmission electron microscope (TEM) and electron back scatter diffraction (EBSD) at the gauge section. The experimental results show that the size of the rib-like α′- martensite formed during tensile testing is sensitive to temperature but insensitive to cold rolling reduction. The results also show that tensile strength is more sensitive to temperature than yield strength, while the effect of cold rolling is completely opposite. The cold rolling inhibits the martensite transformation kinetics of the tensile process after promoting it at a certain temperature condition, and the promotion effect reaches a maximum value approximately 4% when the cold rolling reduction is 10%. However, it also suppresses the α′- martensite content finally formed during tensile testing due to the high stress-strain state when the cold rolling reduction exceeds 10%, while a larger cold reduction promotes the production of α′- martensite. In addition, lowering temperature can effectively promote the formation of e-martensite at the end of the Stage I of work hardening, and becomes a transitional phase of subsequent transformation to α′- martensite, which directly leads to the enhancement of strength in the stage II of work hardening.

Heterogeneous nanograined structures have been proposed to achieve unprecedented mechanical properties. The objective of the present work is to investigate fatigue crack growth behavior of harmonic structure designed austenitic stainless steels at room temperature. The harmonic structured SUS316L steels were produced by mechanical milling and subsequent hot isostatic pressing. Fatigue crack propagation tests were carried out under load ratio from 0.1 to 0.5. Results show that the change in fatigue crack growth rates of harmonic structured SUS316L steels is insignificant even as grain size decreases. The load ratio has little influence on the fatigue crack growth rates for harmonic structured SUS316L steels compared to their coarse-grained counterparts. It is worth mentioning that fatigue crack growth is impeded by embedded coarse grains, which results in fatigue crack deflection and secondary cracks. The continuously network shell (UFG structure) provides increased tensile strength, enhancing the fatigue crack nucleation resistance. Simultaneously, the embedded core (CG structure) restrains crack growth. Therefore, the harmonic structure design is proposed to be an effective method to achieve good balance of high strength and high ductility as well as good fatigue resistance.

In this work, dissimilar rapid Rotary Friction Welding of WC-Co cermet to AISI 304 L austenitic stainless steel has been conducted using different friction times. The microstructural examination showed that the increase in friction time from 4s to 12s increases the grain size in both the heat affected zone and the thermo-mechanically affected zone and enlarges the extent of the fully dynamically recrystallized zone. EDS analysis revealed the existence of a Fe Cr W rich band along the WC-Co/AISI 304 L interface in the central region of the weld joint and its absence from the peripheral region. The formation of this band suggests the occurrence of a mutual inter-diffusion between the cermet and the steel which enhanced the metallurgical bonding of the interface. The mechanical behavior investigated by nano-indentation measurements and nano-scratch tests revealed that, regardless the friction time effect and considering the 304 L ASS side, the highest hardness (HIT) and the lowest Young's modulus (EIT) values were recorded in the fully dynamically recrystallized zone. Besides, the increase of friction time resulted in an increase of hardness and Young's modulus of each zone in the AISI 304 L steel side.

In situ synchrotron X-ray diffraction was used to track real-time austenite reversion in AISI 201 austenitic stainless steel deformed to a true strain of 0.34 under tensile testing. The deformed material was continuously annealed from 100 °C up to 800 °C at a heating rate of 0.05 °C s−1. Phase changes and microstrain partitioning were evaluated by means of the direct comparison method and modified Williamson-Hall plots, respectively. The microstructure of the deformed steel consists of γ, e- and α′-martensite with volume fractions of 0.24, 0.07, and 0.69, respectively. e → γ reversion occurs within the temperature range of 150–400 °C through a shear mechanism. The starting (As) and finishing (Af) temperatures for α’ → γ reversion are 486 °C and 770 °C, respectively. Three stages were distinguished for this reaction. By evaluating the crystallite size of both γ- and α′-phases, it can be inferred that α' → γ reversion is diffusion-controlled. Such results were corroborated by thermodynamic simulations to assess the driving force for γ-formation. Microstructural aspects such as γ-nucleation sites, in-grain misorientation, grain refinement, and crystallographic texture were investigated by means of electron backscatter diffraction. Fresh formed ultrafine austenite grains hold different crystallographic orientations than those of untransformed austenite.

This study investigated stress corrosion cracking (SCC) nucleation in sensitized austenitic stainless steel from the viewpoint of grain boundary (GB) character and strain at a GB where SCC can nucleate. Prior to SCC testing, normal and shear strains were measured on the surface of a specimen subjected to an initial strain of 1% via the digital image correlation (DIC) technique. Then, SCC testing was conducted under a constant load corresponding to the initial strain in a tetrathionate solution. After testing, crystal orientations in the specimen surface were measured by the electron backscattered diffraction (EBSD) technique. The relationship among crack initiation sites, GB character measured by EBSD, and strains measured by DIC is discussed. Stress corrosion cracks were found to be mainly initiated at random large-angle boundaries. The sites of intergranular crack initiation were characterized using the maximum normal strain along GBs, while the shear strain did not affect the crack initiation.

In-phase thermomechanical fatigue tests with various mechanical strain amplitudes (0.6% to 1.2%) at temperature interval of 250–450 °C were carried out on 316LN austenitic stainless steel. The principal deformation and damage mechanisms include dynamic strain aging (DSA) and oxidation. The operation temperature range of DSA is about 400–600 °C determined by monotonic tensile tests. The distinct serrated flow in hysteresis loops and significant cyclic hardening are representative manifestations of the occurrence of DSA which promotes formation of well-developed planar slip bands and intense persistent slip markings (PSMs) characterized by surface extrusions and intrusions. Conspicuously localized oxidation at sensitive grain boundaries and along PSMs is observed with an iron oxide composition. Moreover, the degree of oxidation is positively correlated to strain amplitude. The oxidation plays a significant role in microcrack nucleation and growth which is further enhanced by the strong interaction between oxidation and DSA. The oxidation-assisted intergranular cracking is controlled jointly by a combination of effects including thermal mismatch, slip bands impingement and deformation incompatibility while two different types of transgranular cracks along oxidized PSMs are proposed depending on the particular locations where brittle oxide cracking occurs.

This study used crystallographic and mechanical analyses to investigate the nucleation of intergranular stress corrosion cracking (IGSCC) on a smooth surface of type 304 austenitic stainless steel. Constant load testing was conducted on thermally sensitized austenitic stainless steel in a tetrathionate solution, and the nucleation behavior of IGSCC on a smooth surface was observed in situ. Then, grain boundaries (GBs) at which stress corrosion cracks occurred were characterized based on the coincidence site lattice model, GB length, and stress acting on GBs. It was found that GBs with the following characteristics exhibited high susceptibility to IGSCC: GBs with little resistance to intergranular corrosion (IGC), long GBs, and GBs subjected to high normal stress. Shear stress acting on GBs seemed to have little effect on the nucleation of IGSCC. Then, a criterion for intergranular stress corrosion crack initiation was developed using the susceptibility of GBs to IGC, GB length, and the stresses acting on GBs.

In this paper, we proposed an inverse gradient nanograined structure (IGNS) to improve the mechanical behavior of an austenitic stainless steel. Using high strain cold rolling and subsequent ultra-high-frequency electromagnetic induction heating, we successfully fabricated an IGNS layer on AISI 316L stainless steel, within which the grain size exponentially decreases from the micrometer scale in the surface to the nanometer scale in the inner center. Microstructure evolution was systematically characterized by transmission electron microscopy. The uniaxial tensile test result shows a superior strength–ductility synergy in the IGNS sample. It obtained a yield strength of 842 MPa and a uniform elongation of 16%. A linear relationship between yield strength and uniform elongation was exhibited in IGNS samples with different volume fractions. Effects of the IGNS layer on strength and ductility were examined. Additionally, the effect of recrystallized grain distribution on mechanical properties was discussed.

Understanding serrated yielding behavior resulting from dynamic strain aging (DSA) is essential for design and safety considerations. In this work, uniaxial tensile tests were carried out at temperatures ranging from 298 to 1073 K and strain rates 10−5 – 10−3 s−1 followed by microstructural examination of Fe-25Ni-20Cr (wt%) austenitic stainless steel (Alloy 709), a candidate structural material for Sodium-cooled Fast Reactors. Serrated yielding was found to occur in this alloy in two temperature regimes; low-temperature serrated flow (LT-SF) from 498 to 648 K and high-temperature serrated flow (HT-SF) from 648 to 973 K (depending on the strain rate) with activation energies of 103 ± 13 kJ/mole and 204 ± 11 kJ/mole respectively. The critical strain for the occurrence of serrations was found to increase with strain rate as an exponential function with exponent (m + β) of 0.78 ± 0.1 and 1.56 ± 0.2 for the LT-SF and HT-SF regimes respectively. Based on the activation energies and m + β values, diffusion of interstitial atoms has been suggested to be responsible for serrated flow in the LT-SF regime while Cr atom migration was inferred to be responsible for DSA in the HT-SF regime. Manifestations of DSA in the Alloy 709 were observed including peaks and/or plateaus in flow stresses along with negative strain rate sensitivity at intermediate temperatures. However, no loss in ductility was observed within DSA regime attributed to the relatively high work hardening rate and strain-hardening exponent. The samples deformed in DSA regime showed planar substructure while equiaxed subgrains formed at higher temperatures. The fraction of low angle grain boundaries after deformation exhibited a valley at intermediate temperatures believed to be another manifestation of the DSA.

The present work aims to compare the residual strain fields of the austenitic stainless steel 316L samples under specific loading conditions, obtained by different techniques. Experimentally, Digital Image Correlation (DIC) technique is used to track the (residual) strain maps during the specimen loading. Microstructure-based Finite Element (FE) models based on two different approaches for the material behavior (Crystal Plasticity (CP) and SNF) provide the numerical results. The quality of the outcomes reinforces the use of a combination of these techniques to study the residual strain locations in the microstructure, as well as its evolution during the deformation process. Whereas the results based on CP and SNF showed good agreement, some minor differences between calculated and measured (DIC) strains could be observed.

A fine-grained AISI 304 austenitic stainless steel (ASS) was fabricated by cryogenic-rolling and cycle annealing. The fine-grained ASS was pre-strained to various strains of 0–0.16, and the corresponding microstructures, kinetics of strain-induced α′-martensite (SIMα′) and work-hardening behavior were investigated based on quantitative characterization, interrupted-tensile tests and physical metallurgy. We find that the yield strength of the ASS can be enhanced by combining grain-refining with pre-straining due to the strengthening effect of grain-refinement, dislocations, shear bands and SIMα′. The formation rate of shear band increases with increasing pre-strain (ep) and the probability for an intersection of shear band to form a SIMα′ embryo is insensitive to the pre-strain, and as a result, the kinetics of SIMα′ is accelerated by pre-straining. The fine-grained ASS with ep ≈ 0–0.16 display a three-stage work-hardening behavior, and the corresponding Stage I, II and III are completed in advance mainly due to their faster kinetics of SIMα′ and severer dynamic recovery caused by pre-straining. The work-hardening rate (Θ) of pre-strained fine-grained ASS decreases faster than that of non-pre-strained counterpart at Stage I, accompanying with dislocations-dominated hardening. The Θ ascend with increasing strain to a peak value at Stage II and then decline continuously to necking at Stage III, following as SIMα′-governed hardening, which can be tailored by grain-refining and pre-straining. Our studies provide the direct experimental evidence for the pre-straining to trim the kinetics of SIMα′ in a fine-grained ASS. These observations help to deepen understanding of the effect of pre-strain on the mechanical behavior of other fine-grained strengthening ASS and transformation-induced-plasticity (TRIP) assisted alloy.

High borated stainless steel sheets have been widely applied in the nuclear power industry because of good thermal neutron absorption property. However, the large and network-like borides existing around the austenite grains in the conventional ingot casting steel seriously deteriorate the hot workability and the mechanical properties at ambient temperature. In this work, we tried to acquire ultra-fine microstructure by sub-rapid solidification using a novel twin-roll strip casting technology so as to enhance the mechanical properties of 2.1%B austenitic stainless steel. Surprisingly, dispersive and very fine borides mostly smaller than 5 µm were produced in the as-cast microstructure without network-like distribution. The morphologies and crystal structure of borides as well as the stacking faults in borides were investigated in detail. After subsequent hot-rolling and solution treatment, ultra-fine borides were observed with more than 50% of which in a size range of 0.3–1.5 µm, significantly smaller than those of the conventional ingot casted and hot rolled steel. Benefiting from the ultra-fine borides, excellent mechanical properties which had not been reported were obtained. In particular, a total elongation of 14.1% was exhibited, which was twice as high as that of the conventional ingot casted and hot rolled steel. Thus a new structure-function combining high borated stainless steel sheet was achieved. The strengthening and plasticity increasing mechanism was discussed based on Hall-Petch relationship, Orowan mechanism and strain-hardening rate analysis. The fracture behavior in tensile deformation was studied in detail. A characteristic fracture process accompanied with the initiation and coalescence of cavities in austenite matrix was found. This work not only developed a new processing way to produce high borated stainless steel with excellent mechanical properties, but also provided a potential solution for some other hard-worked metallic materials with brittle eutectic phase.

Electron backscatter diffraction (EBSD) was employed to establish microstructure evolution in type 321 metastable austenitic stainless steel during rolling at a near-liquid-nitrogen temperature. A particular emphasis was given to evaluation of microstructure-strength relationship. As expected, cryogenic rolling promoted strain-induced martensite transformation. The transformation was dominated by the γ→α′ sequence but clear evidence of the γ→e→α′ transformation path was also found. The martensitic reactions were found to occur almost exclusively within deformation bands, i.e., the most-highly strained areas in the austenite. This prevented a progressive development of deformation-induced boundaries and thus suppressed the normal grain-subdivision process in this phase. On the other hand, the preferential nucleation of martensite within the deformation bands implied a close relationship between the transformation process and slip activity in parent austenite grains. Indeed, the martensite reactions were found to occur preferentially in austenite grains with crystallographic orientations close to Goss {110} and Brass {110} . Moreover, the martensitic transformations were governed by preferential variant selection which was most noticeable in e-martensite. The sensitivity of the martensitic reactions to the crystallographic orientation of the austenite grains resulted in re-activation of the transformation process after development of a deformation-induced texture in the austenitic phase at high strains. Both martensitic phases were concluded to experience plastic strain which resulted in measurable changes in misorientation distributions. Cryogenic rolling imparted dramatic strengthening resulting in a more-than-sixfold increase in yield strength. The main source of hardening was the martensitic transformation with lesser contributions from dislocations and subboundary strengthening of the austenite.

Selective Laser Melting (SLM) was applied to Additive Manufacturing (AM) of carbide ceramic particulate reinforced 316 L stainless steel matrix composites. The dense and ultrafine-grained Austenite matrix with uniformly distributed and nano-sized VCx particulate reinforcements were obtained according to the tailored SLM process parameters and 3 wt% submicron-sized V8C7 ceramic particle addition into 97 wt% 316 L powder via ball-milling treatment. The influences of SLM process parameters on the SLM printed composites in terms of density, surface roughness, phase constitution, microstructure characteristics and mechanical tensile performance were investigated in detail, comparing with the SLM printed 316 L in absence of vanadium carbides. Based on the electron back-scattered diffraction analyses, the number of grains with grain size less than 2 µm accounted for more than 90% in the printed composites. The formation mechanisms of the printed composites with their unique microstructure characteristics were elaborated. The room-temperature ultimate tensile strength of the printed 316 L matrix composites achieved higher than 1400 MPa, which was more than twice as high as that of printed 316 L stainless steel. It contributed substantially to the strengthening effect on the printed composites of the ultrafast laser-induced and highly nonequilibrium melting-solidification nature of SLM process, the dispersed carbide ceramic particulate reinforcements, the nanoscaled nucleation sites of VCx for the equiaxed Austenite grains, and the inhibition against Austenite matrix growth by particulate dispersedly distributed along Austenite grain boundaries.

In this work, the role of prior plastic deformation and rate of deformation on the corrosion behavior of plastically-deformed metastable AISI 321 austenitic stainless steel in 3.5 wt% NaCl solution was investigated. Samples were deformed under dynamic (έT = 8.8 × 103 s−1) and quasi-static (έT = 4.4 × 10−3 s−1) loading conditions to a common true strain of ~0.86 using a split Hopkinson pressure bar and Instron R5500 mechanical testing systems, respectively. While the specimen subjected to quasi-static compression experienced substantial deformation-induced martensitic transformation, phase transformation was significantly suppressed in the specimen subjected to dynamic loading due to temperature rise during impact. The yield strength of the specimen under dynamic impact loading is higher than that of the specimen compressed under quasi-static loading condition. However, the specimen deformed under quasi-static loading shows higher strain-hardening rate compared to the specimen subjected to dynamic impact load which is dominated by thermal softening at true strain beyond ~0.6. Using the Tafel polarization and impedance spectroscopic techniques, the deformed specimens were found to be more resistant to chloride-induced intergranular corrosion at room temperature compared to the undeformed ones. The specimen subjected to quasi-static loading exhibits significantly higher corrosion resistance in comparison with those subjected to dynamic loading. This could be attributed to the formation of a denser passive film on the specimen subjected to quasi-static loading on exposure to the test electrolyte over time. In addition to the role of particles on pitting formation in metals exposed to corrosive environment, the current work affirms the important role of prior plastic deformation on the corrosion behavior of the investigated alloy. Evolution of twins and stable-end orientation (preferred crystallographic orientation) in both parent (austenite) and product (martensite) phases nullified what would have been the negative effects of plastic deformation (due to the associated deformation-induced martensitic phase and stored defects) on the corrosion resistance of the investigated stainless steel.

The deformation behaviors of 310s stainless steel weld metals (WMs) with different Nb contents are investigated under uniaxial tension at temperatures ranging from ambient temperature to 1000 °C. The results show that Nb effectively improves the strength of the WMs in the whole test temperatures range. The addition of Nb causes a change in the fractures of the WMs from intergranular brittle fractures to intragranular ductile fractures at elevated temperature. Hence, elemental Nb enhances the elevated temperature plasticity of the WMs. However, as the eutectic Nb(C, N) promotes the nucleation and propagation of the cracks, Nb decreases the elongation of the WMs at temperature below 700 °C. With the increase in the deformation temperature, the strength of the WMs decreases monotonously. However, the elongation of the WMs shows a nonmonotonic relationship with the temperatures. The deformation twins can occur at room temperature and improve the strength and elongation of the WMs. Meanwhile, the cracks nucleate around the eutectic Nb(C, N) at 1000 °C, leading to the minimum elongation of the Nb-bearing WMs in the whole test temperature range. The differential scanning calorimetry result suggests that the melting of the eutectic Nb(C, N) may be the dominant reason for the nucleation of the cracks and result in the reduction in the elongation of the Nb-bearing WM at 1000 °C.

The austenitic stability of selective laser melted austenitic stainless steel was investigated by the tensile tests in the 80–300 K temperature range. The selective laser melting process greatly improved the yield strength, but severely suppressed the stain-induced α′ martensitic transformation compared with the conventional austenitic stainless steel. The high density of low-angle grain boundaries and fine cellular microstructures were observed in the selective laser melted austenitic stainless steel, which suppressed dislocation slip and deformation twinning during deformation, resulting in the reduction in the nucleation sites of α′ martensite and the enhancement of the austenitic stability.

Usually, the martensite-to-austenite transformation in steels results in volumetric contractions. However, we observed that the martensite-to-austenite transformation induced expansion in rolling direction of heavily cold-rolled metastable austenitic stainless steel during heating. Herein, we provide a mechanistic insight into the anomalous dilatometric behavior.

A systematic investigation on grain growth behavior, grain boundary character distribution and mechanical properties of Fe-21Cr-15Ni-6Mn-Nb non-magnetic stainless steel was performed after solution treatment in temperature range of 1100–1200 °C with various durations. The results showed the hot-rolled plate still maintained fine microstructure with the grain size range of 11.5–19.6 µm after solution treatment. A Nb(C, N) drag mechanism concurrent with a restraining effect of Σ3n boundaries during solution treatment resulted in a slow kinetics with grain growth exponent n of ~ 4. The proliferation mechanism of Σ3n boundaries mainly varied from Σ3 regeneration to new twinning accompanied by grain growth. Twin boundary has an effective strengthening role like a grain boundary. The YS, UTS and elongation of solution-treated plate was above 634.0 MPa, 838.9 MPa, 45.3％, respectively, which were superior to the conventional austenitic stainless steel. The Hall-Petch relationship was determined well between the grain size under the condition of either including or excluding twin boundaries and tensile results. An inherent relationship between k ′ and k ( k ′ and k correspond to the circumstance of including and excluding twin boundaries, respectively), was expressed by k ′ = 1.52037 + 1.67168 k .

High-Energy Diffraction Microscopy (HEDM) was employed to measure and compare the evolving micromechanical state of two alloys, an austenitic stainless steel (316 L) and nickel-based alloy (Inconel 625) fabricated by both conventional methods and powder metallurgy with hot isostatic pressing (PM-HIP) during in situ uniaxial tensile testing. Each of the four materials was tested through the elastic regime to just beyond yield. HEDM was performed at room temperature in the far-field (ff) configuration at the Cornell High Energy Synchrotron Source to measure grain-average elastic strains and subsequently derive stress tensors. The evolution of the normal stress component along the loading direction in individual grains as a function of macroscopic deformation is presented. Initially, grain-scale stresses in the loading direction are more heterogeneous in the wrought alloys than in the PM-HIP alloys. Notably, many peripheral grains in the wrought specimens are near yield even before load is applied. With increased loading, grain-scale stresses tend to homogenize in all specimens. Orientation fields measured using electron back scatter diffraction (EBSD) are used to determine grain morphologies and interpret the ff-HEDM data. The PM-HIP grains tend to be finer and rounder in shape than the wrought grains, potentially explaining the grain-scale stress distributions. Finally, yield strength and modulus of elasticity are measured for the four alloys and correlated to the resultant grain size and morphology from the fabrication processes.

This study presents a thorough characterization of the creep properties of austenitic stainless steel 316L produced by laser powder bed fusion (LPBF 316L) contributing to the sparse available data to date. Experimental results (mechanical tests, microscopy, X-ray computed tomography) concerning the creep deformation and damage mechanisms are presented and discussed. The tested LPBF material exhibits a low defect population, which allows for the isolation and improved understanding of the effect of other typical aspects of an LPBF microstructure on the creep behavior. As a benchmark to assess the material properties of the LPBF 316L, a conventionally manufactured variant of 316L was also tested. To characterize the creep properties, hot tensile tests and constant force creep tests at 600 °C and 650 °C are performed. The creep stress exponents of the LPBF material are smaller than that of the conventional variant. The primary and secondary creep stages and the times to rupture of the LPBF material are shorter than the hot rolled 316L. Overall the creep damage is more extensive in the LPBF material. The creep damage of the LPBF material is overall mainly intergranular. It is presumably caused and accelerated by both the appearance of precipitates at the grain boundaries and the unfavorable orientation of the grain boundaries. Neither the melt pool boundaries nor entrapped gas pores show a significant influence on the creep damage mechanism.

The present paper aims to contribute to the understanding of the process-(micro)structure-property linkage in additively manufactured materials. The scanning pattern effects on the microstructure and elastic properties of 316L austenitic stainless steel produced by the powder bed based additive manufacturing are explored. A multiphysics methodology adapted for this purpose combines the physically-based cellular automata for simulation of the grain structure evolution, finite-difference heat transfer calculations in a simplified way following the concept of a moving melt pool, and elastic anisotropy modeling based on the orientation distribution functions and Reuss, Voigt, and Hill schemes. Unidirectional and bidirectional scanning strategies strongly influence the texture and elastic properties of as-built components. The grain structures obtained are characterized by bi-component crystallographic textures. While the specimen printed using a bidirectional strategy demonstrates the major { 011 } 100 Goss and minor { 001 } 100 cube texture components, the unidirectional scanning pattern leads to the rotation of both grain arrangement in space and crystallographic texture. The specimen produced with a unidirectional strategy tends to have coarser grains, more severe texture and more pronounced anisotropy of elastic properties than that printed using a bidirectional scanning pattern. The latter is concluded to exhibit the orthotropic behavior based on the calculated stiffness tensor. In addition, the present study reports stiffness tensors for the additively manufactured steel components.

In the present research, the AISI 321 austenitic stainless steel was subjected to cold rolling with different amount of reductions. The 80% cold rolled samples were then annealed in the temperature range of 700–900 °C with different durations. The microstructural evolutions during cold rolling and subsequent annealing were investigated by XRD analysis, optical and scanning electron microscopy, and feritscope analysis. The mechanical properties were evaluated by room temperature tensile testing. Results showed that the amount of strain induced martensite is increased with increasing the rolling reduction. Annealing at 700 °C for 2 min resulted in the simultaneous increase of tensile strength and elongation due to the formation of ultrafine grain austenite. By annealing at 900 °C for 5, 10, and 15 min the bimodal grain structure was obtained as a result of the occurrence of static recrystallization and martensite to austenite reverse transformation simultaneously. Results of tensile testing showed that the good combination of strength and elongation is obtained by the formation of bimodal grain structure.

The tensile behaviour of austenitic stainless steel after cryorolling and subsequent annealing has been reported. The defect structures generated during cryorolling, lead to the formation of alpha' and epsilon-martensite. Subsequent annealing leads to the formation of refined austenitic grains surrounded by martensitic islands. These martensitic islands consist of alpha' phase only. The dislocation-martensite interaction leads to discontinuous yielding for high Ultimate Tensile Strength (UTS)/Yield Strength (YS) ratio. In case of annealing at 923 K (650 degrees C) for 10 min after 30% cryorolling leads to nearly 30% untransformed martensite whereas only 7% martensite remains after annealing for 1 h. The effect of remnant martensite on tensile behaviour leading to yield plateau has been discussed in this work. It can be noticed that sample with 20% cryo rolling and annealed at 650 degrees C for 1 h shows higher YS (721 MPa) with YS/UTS ratio of 0.79 and similar to 18% uniform elongation. Sample with 30% cryo rolling and annealed at 650 degrees C for 1 h shows YS of 305 MPa with YS/UTS ratio of 0.27 and uniform elongation of similar to 24%. Therefore, 20% cryo rolled and annealed at 650 degrees C for 1 h has the best combination of mechanical properties.

The relationship between the intermetallic compound (IMC) layer thickness including the deviation and joint strength was evaluated for the dissimilar joints between aluminum alloys and austenitic stainless steel. To evaluate the interface with various IMC layer thicknesses, the joints by friction stir welding were solution-heat-treated and artificially aged. The transitions of joint strength and fracture position were characterized for the IMC layer thickness ranging from 0 to 2.5 µm. Further, we demonstrated that the fracture position can be determined by the percentage of the joining area via the IMC layer thickness below a threshold value, which was 0.4 µm.

The effect of grain size on high-cycle fatigue behavior under axial forces in the 18Cr-8Ni austenitic stainless steel was investigated. The fatigue strength of the austenitic stainless steel, obtained by the phase reversion process, with grain sizes of 400 nm, 1.4 µm, and 12 µm were 811 MPa, 568 MPa, and 501 MPa, respectively. After cyclic deformation, remarkable slip bands were formed on the surfaces of coarse-grained (CG) and fine grained (FG) samples, whereas, no slip bands or other deformation marks were observed on the surface of the nanograined/ultrafine-grained (NG/UFG) sample. Randomly oriented cracks of a few micrometers long appeared on the CG sample, while the cracks on NG/UFG were larger and the propagation direction was perpendicular to the loading direction. The increase in hardness after cycling was inversely proportional to the increase in strain-induced martensite among the NG/UFG, FG, and CG samples. Strain-induced martensite was produced primarily at the grain boundaries or slip bands in CG steels; in contrast, martensite was concentrated in the vicinity of nanograins in FG and NG/UFG steels. After fatigue, in the CG sample, a small amount of martensite, deformation bands, and large number of dislocations were formed, while in the NG/UFG sample a very small amount of e-martensite, strain-induced martensite, and dislocations were formed.

The effect of heating rate on the mechanism of reverse transformation from martensite to austenite during continuous heating process was studied in a cold-rolled 304 stainless steel, and the corresponding mechanical properties were analyzed. It indicated that the reverse transformation occurred diffusionally at slow heating rate range ( 40 °C/s) resulted the reverse transformation to occur via martensitic shear-type. At medium heating rates (10–40 °C/s), the reversion mechanism gradually changed from diffusional to shear-type. The diffusional reversed austenite was characterized by equiaxed, defect-free, nano/ultrafine grains with grain size of 100–500 nm, while the martensitic shear-type reversed austenite exhibited lath-type/banded grains with high defects density. At 100 °C/s, an excellent combination of high-strength and high-ductility with significant work hardening ability was obtained. Moreover, the product of strength and ductility was enhanced over three times from 11.14 GPa (at 2 °C/s) to 35.78 GPa (at 100 °C/s). The strain gradient due to the heterostructure, the glide of preexisting mobile dislocations and the controlled release from TRIP effect contributed to the improved mechanical properties. The study also indicated that reversion mechanism had no effect on austenite texture.

In the present research, the hot deformation microstructure of AISI 321 austenitic stainless steel was studied by performing hot compression tests in the temperature range of 800–1200 °C and strain rates of 0.001–1 s−1. Microstructural evolutions were studied using transmission and scanning electron microscope equipped with EBSD camera. Results show that the main restoration process in the temperature range of 800–950 °C is dynamic recovery and dynamic recrystallization is the prevailing softening mechanism in the temperature range of 1000–1200 °C. TEM observations confirm the occurrence of dynamic recovery and recrystallization at the specified temperatures. Increasing deformation temperature leads to the increase in the fraction of high angle boundaries and simultaneous decrease in the fraction of low angle boundaries. Also, deformation at 800 and 850 °C is accompanied by the formation of M23C6 precipitates at austenite grain boundaries that increases the deformation activation energy and results in grain boundary sensitization detected using scanning electron microscopy.

Welding of dissimilar metals is challenging, particularly between crystalline metals and metallic glasses (MGs). In this study, Zr 65.7 Cu 15.6 Ni 11.7 Al 3.7 Ti 3.3 (wt%) MG structures were built on 304 stainless steel (SS) substrates by laser-foil-printing (LFP) additive manufacturing technology in which MG foils were laser welded layer-by-layer onto the SS substrate with a transition route, i.e., SS → V → Ti → Zr → MG. The direct welding of MG on SS would lead to the formation of various brittle intermetallics and the consequent peeling off of the welded MG foils from the SS substrate, which could be resolved via the use of V/Ti/Zr intermediate layers. The chemical composition, formed phases, and micro-hardness were characterized in the dissimilar joints by energy dispersive spectroscopy, X-ray diffraction, and micro-indentation. Since the intermediate materials were highly compatible with the base metals or the adjacent intermediate metals, undesirable intermetallics were not detected in the dissimilar joint. The bonding tensile strength between the SS substrate and the MG part with intermediate layers was measured about 477 MPa.

Contrary to the commonly observed embrittlement during dynamic strain aging, we report in this note distinct enhancement in ductility in a Nb-containing and nitrogen stabilized Fe-25(wt%)Ni-20Cr austenitic stainless steel (Alloy 709) at temperatures from 623 K to 873 K at 10−4 s−1 where serrated flow is noted. This observation is rationalized in terms of the influence of strain hardening parameters and strain rate sensitivity on uniform elongation and ductility respectively.

We present the evolution of crystallographic orientation and microstructure during thermomechanical fatigue (TMF) of heat-resistant cast austenitic stainless steel at peak temperatures reaching 950 °C using the electron backscatter diffraction (EBSD) method. Higher restraints or peak temperatures induced larger crystal misorientation by geometrically necessary dislocations (GNDs), forming dislocation walls or subgrains in the grains. Networked carbide clusters in the microstructure locally amplified the misorientation in the adjacent matrix and initiated fatigue cracks. The mean value of cumulative misorientations over a specific distance in the matrix was linearly proportional to the cyclic plastic strain.

Ultrafine grained austenitic stainless steel, with two different types of grain size distributions, was studied for tensile deformation behavior. Tensile deformed specimens were analyzed by electron backscatter diffraction using scanning electron microscope. It was found from this study that uni-modal grained stainless steel (SS) having a larger fraction of submicron grains exhibited early fracture, which is attributed to the development of extensive strain localization. On the other hand, the microstructure of SS having bimodal grain size distribution showed a good combination of strength and ductility. EBSD analysis of the deformed region of these two samples revealed the presence of a distinct transition zone between undeformed or less deformed and extensively sheared regions. Multiple micro shear bands were found to be associated with the transition zone of unimodal type microstructure. The micro shear bands seen in the transition zone of unimodal SS led to the development of strain-induced martensite (SIM), which, in turn, is helpful in delaying the strain localization. However, in bimodal grained SS, the larger fraction of micron size grains undergoes a shape change, with a rotation towards [112] orientation, which results in the formation of a larger fraction of SIM having [112] orientation. The propensity for development of high SIM was found to prevent strain localization in bimodal grained SS.

304 austenitic stainless steel was precharged with hydrogen (∼ 0.3 at%) and subsequently slow strain rate tensile tested at room temperature. The motivation of this investigation was to further clarify the nature of crack initiation, and especially the role of martensitic transformation, in metastable austenitic stainless steel under the influence of hydrogen. However, it must be distinguished between martensite formed by pre-straining, i.e. γ-α′-transformation prior to the exposure to hydrogen and γ-α′-transformation during plastic deformation under the influence of hydrogen. In the first scenario martensite contents formed by pre-straining could not be correlated with HE effects in tensile tests. In the second scenario crack initiation could always be attributed to locations with local strain incompatibilities like twin-grain boundary triple junctions, γ-α′ phase boundaries, α′ martensite, δ-ferrite stinger and strain incompatibility sites within γ grains. The results clearly show that γ-α′ transformation only plays a role in two of the five failure modes indicating that γ-α′ transformation is not the sole root cause for the high susceptibility of 304 stainless steel to hydrogen embrittlement.

The objective of the present study is to obtain austenitic stainless steel with super-high yield strength and good ductility through a heterogeneous nano/ultrafine-grained design, fabricated by the reverse transformation of strain-induced martensite and the recrystallization of deformed austenite using two cold-rolling and annealing processes. The second cold-rolling and annealing process was performed on a bimodal microstructure obtained by the first cold-rolling and annealing process on the original microstructure. The bimodal microstructure consists of nanometer grains and a certain quantity of micrometer grains distributed in the lamella. The results of this study demonstrate that after the second process, both the coarse-grain and fine-grain areas of the bimodal microstructure formed their own finer bimodal microstructure and the lamella of the grain distribution became narrower. The yield strength (1221 MPa) and tensile strength (1376 MPa) of the heterogeneous nano/ultrafine-grained steel were greatly improved in the situation where the total elongation still remained at the high level of 45.3%. The fine-grained strengthening, back-stress strengthening, twinning induced plasticity (TWIP) effect and transformation induced plasticity (TRIP) effect were produced by the heterogeneous nano/ultrafine-grained microstructure during tensile possess synthetically contributed to the high strength and ductility.

An AISI 304 austenitic stainless steel (ASS) with various ultrafine- or fine-grained structures was fabricated by cold- or cryogenic-rolling and annealing. The microstructure and mechanical properties of the ultrafine- or fine-grained ASS were investigated based on statistical data and physical metallurgy. The results showed that much more volume fraction of α′-martensite can be obtained by cryogenic-rolling in comparison with cold-rolling under a similar rolling strain, and e-martensite was a medium to transform into α′-martensite finally during cryogenic-rolling. The deformed ASS with larger volume fraction of α′-martensite was beneficial to obtaining finer structure with a narrow distribution of grain sizes after the similar annealing process. The cycle annealing was a feasible method to make reverse transformation completely and to inhibit the structural coarsening simultaneously for the cold- or cryogenic-rolling ASS. The yield strength was enhanced by cryogenic-rolling and cycle annealing to be approximately 2.7 times higher than that of solution-treatment state. The tensile strength was not changed evidently, and the uniform strain was apparently decreased with reducing grain size. There is no significant relevance between the mechanical stability of austenite and average grain size for the ultrafine- or fine-grained ASS; however, their mechanical stability was less than that of solution-treatment state.

Laser welding is attractive for numerous applications requiring materials joining, due to its low energy input. However, it is unknown how the laser welding process influences the microstructure-property relationship across the weldment. The objective of this study is to determine the strengthening mechanisms in the weld metal, base metal, and heat affected zone (HAZ) of an AISI 304/308L stainless steel (SS) laser weldment. Scanning electron microscopy (SEM) with electron backscattered diffraction (EBSD) scanning and transmission electron microscopy (TEM) was used to evaluate the microstructure, and static nanoindentation was used to evaluate strength across the weldment. Although the HAZ has a finer dendritic grain structure, its higher hardness compared to the base and weld metal cannot be explained by the Hall-Petch relationship. Therefore, a new strengthening model for weldments that considers the evolution in grain boundary size and orientation angle, as well as dislocation density, precipitation, and solid solution is proposed. Notably, core-shell Ti-C-N precipitation, which provides Orowan dislocation bypass strengthening, is found to be a major contributor to strengthening in the HAZ. The proposed model predictions fall within 10% of experimentally measured properties for all three regions of the analyzed weldment.

Creep deformation of an advanced high-strength, creep-resistant austenitic stainless steel Sanicro 25 was investigated experimentally at temperatures from 700 °C to 800 °C and stresses below 40 MPa. Results show that the creep behaviour is qualitatively similar as that of the other creep-resistant steels, having much weaker stress and temperature dependencies of the creep rate than those at higher stresses. Creep performance of the Sanicro 25 steel allows temperatures of at least 50 °C higher than the classical AISI-316 H austenitic steel, even at low stresses.

The paper reports a substantial improvement of the static and cyclic strength of a Cr-Ni-Ti austenitic stainless steel nanostructured by equal channel angular pressing (ECAP). After ECAP at room temperature or 673 K, the mean grain size decreased from 14 µm to 430 nm or 940 nm, respectively; corresponding ultimate tensile strength increased from 610 MPa to 1230 MPa or 940 MPa, and the fatigue limit increased from 275 MPa to 375 MPa or 475 MPa. These enhanced strength properties result from the grain refinement assisted by the intensive twinning in the austenite during ECAP at room temperature and 673 K as well as partial martensitic transformation during ECAP at room temperature. Moreover, the partial martensitic transformation and an increase in the fraction of high angle grain boundaries during subsequent high-cycle fatigue tests were particularly favorable for the improvement of fatigue properties.

The alumina-forming austenitic stainless steel Fe-20Cr-30Ni-2 Nb-5Al was given a solutionizing anneal at 1250 °C followed by anneals at 800 °C for 0, 2.4, 24, 240 h to produce B2 (ordered b.c.c.) and Laves phase precipitates of different sizes with different extents of grain boundary coverage. Both tensile tests and constant-stress (43 MPa) creep tests were performed on the heat-treated materials and on the as-cast alloy at 760 °C. The precipitates grew during the creep testing. In addition, L1 2 (ordered f.c.c.) precipitates nucleated and grew during the creep testing to similar particle sizes after 500 h independent of the prior heat treatment at 800 °C. The specimens given the shortest heat treatment of 2.4 h, which had the smallest initial particle sizes, showed both the highest yield strength and the smallest creep strain after 500 h. The extent of grain boundary coverage by precipitates did not appear to affect the creep rates. No grain boundary cracking or precipitate cracking was found in the heat-treated specimens after creep testing, but the as-cast material failed around 600 h.

The present study aims to investigate the effect of annealing on microstructural evolution and mechanical properties in phase reversion-induced ultrafine/fine-grained 316LN austenitic stainless steel. The commercial 316LN austenitic stainless steel was cold rolled at room temperature to 90% thickness reduction and subsequently annealed in the temperature range of 600–1000 °C for 1–100 min. Evolution of phases in selected samples was identified and quantified by X-ray diffraction together with the corresponding microstructural characterization through optical, scanning and transmission electron microscopy, and electron backscattered diffraction. Mechanical properties of selected samples were determined by the tensile test. The results indicated that 46% α′-martensite and 54% deformed untransformed austenite were obtained in 316LN austenitic stainless steel after 90% cold reduction. Ultrafine/fine austenite grains nucleated at α′-martensite and deformed untransformed austenite via nucleation and growth process on annealing. The average grain size increased gradually with increased annealing temperature and time, with consequent decrease in yield strength and increased elongation.

Advanced scanning transmission electron microscopy (STEM) was used to study two distinct populations of nanoparticles associated with the extraordinary strengthening of the highly-alloyed austenitic stainless steel Sanicro 25 during cyclic loading at 700 °C. Fully coherent and homogeneously dispersed Cu-rich nanoparticles precipitate rapidly as a result of thermal exposure, along with nanometer-sized incoherent NbC carbides that nucleate on dislocations during the cyclic loading at high temperature. The atomic structure of nanoparticles was investigated by probe-corrected high-angle annular dark-field STEM imaging. Compositional analysis of the nanoparticles was conducted using high spatial resolution energy dispersive X-ray spectroscopy combined with electron energy-loss spectroscopy. Experimental observations were validated by image simulations of the Moire-like contrast exhibited by NbC carbides. The important role of both nanoparticle populations for the overall cyclic response is discussed. As a result of pinning effects and associated obstacles, dislocation motion is significantly retarded preventing formation of substructures with lower stored internal energy. With recovery heavily suppressed, forest dislocation strengthening supported by precipitation and solid solution hardening, leads to the remarkable increase of cyclic strength at elevated temperatures.

304 stainless steel is known to be metastable as the austenite phase can transform into martensite under deformation. In this work, both ex-situ and in-situ transmission electron microscopy (TEM) characterization were used to investigate the mechanisms of the deformation-induced transformation at room temperature. The ex-situ tensile tests were conducted at a strain rate of 10−3 s−1 until rupture, followed by TEM and X-Ray Diffraction (XRD). Samples were also interrupted at strains of 7%, 18%, and 30% with the goal of investigating the intermediate microstructure. In addition, tensile tests were conducted in-situ in a TEM at 25 °C using a special straining-stage with the goal of capturing the nucleation and growth of the martensitic phase as it develops under deformation. The formation of stacking faults and the subsequent formation of e-martensite (hcp) through their overlapping/bundling was captured in-situ, confirming the role played by Stacking Faults (SFs) as intermediate step during the transformation from γ-austenite to e-martensite. Direct transformation of γ-austenite (fcc) to α’-martensite (bcc) was also captured upon straining and characterized. Such unique in-situ observations showcase how in-situ straining in a TEM, as a small scale tensile technique, is a powerful technique to visualize and investigate the mechanisms of deformation induced phase transformations.

The mechanical and corrosion properties of gas metal arc additive manufacturing (GMA-AM) 316L could be optimized by modifying the volume fractions of sigma (σ) and delta-ferrite (δ) phases through heat treatment. Results show that the heat treatment at 1000 °C to 1200 °C for one hour will not obvious influence the morphology of grains in steel but largely influence the contents of σ and δ phases. The heat treatment at 1000 °C effectively increases the amount of σ phase in steel, causing both increase of UTS and YS but decrease of El and RA. The heat treatment at 1100 °C to 1200 °C completely eliminates σ phase, leading to the decrease of UTS and YS but increase of El and RA. The σ phase has better strengthening effect than δ phase, but which may degrade ductility and increase the possibility for cracks generation in steel. Meanwhile, limiting the number of both σ and δ phases through heat treatment can improve the corrosion resistance of steel. And σ phase appears more detrimental impact on degradation the corrosion resistance of steel than δ phase.

The high-temperature tensile behaviors of two 25Cr-20Ni austenitic stainless steels with different V concentration (0 wt% V and 0.3 wt% V, respectively), have been studied at temperature between 200 ℃ and 900 ℃ . The ultimate tensile strength of both steels is strong temperature dependent, which decreases slowly first at 200–300 ℃ , keeps platform then at 300–500 ℃ and decreases rapidly afterwards from 600 ℃ to 900 ℃ . It is caused by the decreasing strain hardening ability, dynamic strain aging and dynamic recovery together with dynamic recrystallization at different temperatures. At higher than 800 ℃ , the elongation of both steels increases markedly due to the dynamic recovery and dynamic recrystallization. However, because of the deteriorated effects of M23C6 precipitates at grain boundary, the elongation of both steels at 700 ℃ does not increase despite decreasing strength. Additionally, the addition of 0.3 wt% V decreases the ductility of the material in the temperature range of 800 ℃ to 900 ℃ , which is induced by the impeding effects of solute vanadium on dynamic recovery and recrystallization.

The effects of electropulsing treatment (EPT) on the microstructure and corresponding mechanical properties of a nano-lamellar 316L austenitic stainless steel were investigated. The original 316L stainless steel features a nano-lamellar framework with high density of dislocations via cold rolling. The nanostructured stainless steel was hardened and strengthened after EPT with relatively low discharge voltage, which may result from the cooperative effect of stable nano-lamellar structure and mobile dislocations recovery. Both the microhardness and tensile strength would decrease significantly with the increase of discharge voltage due to the nucleation and growth of recrystallized grains in the steel.

In the present study, the concept of phase reversion developed by Misra's group (references 2，4,14–16,25) was adopted to process a 304 austenitic stainless steel and explore the microstructural evolution, recrystallization behavior, and relate to mechanical properties. Dual phase structure, including strain-induced martensite and hardened austenite, was obtained by two-stage cold rolling at room temperature. Subsequently, the reversion of martensite to austenite during annealing was carried out and studied in terms of microstructural evolution and recrystallization behavior using a combination of X-ray diffraction, optical microscopy, and electron backscatter diffraction techniques. The Vickers hardness and tensile properties of samples annealed at different conditions were also determined. The X-ray diffraction study indicated that reversion of martensite occurred rapidly at temperatures greater than 750 ℃ because of the change in the reversion mechanism to shear-type reversion. The variation in hardness of annealed samples at 750–850 ℃ exhibited three distinct stages and corresponded to different process. Depending on the grain size and annealing conditions, different combination of yield strength (466–1758 MPa) and ductility (6.6–57.5%) were obtained.

Most of industrial hot deformation processes are performed in variable conditions where the strain rate and/or deformation temperature are not constant. In this work, hot compression tests in both constant and varying strain rate conditions were performed on 304L austenitic stainless steel using a Gleeble 3800 machine. The variations in microstructure and flow stress during and after the transient deformation stage are carefully analysed. It is clearly shown that, following the abrupt increase of strain rate, both the flow stress and substructural changes are subjected to a transient period over strains of ~0.2, before reaching states similar to those developed through constant strain rate conditions at the new strain rate. When the strain rate was rapidly decreased, the flow stress transient stage extended over a lower strain interval than the substructure transient period. It is shown that local misorientation distributions are good indicators of the deformation microstructure of low SFE materials as they capture small variations in deformation structures which cannot be analysed from stress-strain curves alone.